86 SEXUAL REPRODUCTION IN CERTAIN MILDEWS. 



gives us a possible connecting link between the cystocarp and the cseoma 

 type of secidium. It may be that in the cup-shaped secidia branches 

 bearing gametes arise from a true carpogonium, while in indeterminate 

 aecidia of the caeoma type this carpogonial branch has disappeared. 



If the evidence advanced by Mottier and Williams that a reduction 

 division occurs in the development of the tetrasporange be confirmed, 

 it is plain that the homologue of the ascus and teleutospore, and of the 

 basidium as well, is in the tetrasporange and not in the carpospore, as 

 many have been inclined to assume. The carpospores must be inter- 

 preted as conidia intercalated in the life cycle of the sporophyte, just 

 as are the aecidiospores and uredospores in the sporophyte of the rust. 

 In this case a close relationship may be assumed between the aecidium 

 and the cystocarp, each being an asexual stage appearing early in the 

 life history of the sporophyte and having no analogous stage in the 

 sporophyte of the moss or fern. The ascus, on the other hand, would 

 correspond to the tetrasporange, and there would be no stage in the 

 Ascomycetes to correspond to the carpospores of the red algge. We 

 should thus be relieved of the difficulty of assuming relationship between 

 the carpospores produced as buds or chains of cells and the ascospores 

 formed internally by free cell formation. If the tetrasporange is the 

 progenitor of the ascus we must assume that free cell formation, with 

 an abundance of epiplasm, has been developed in the latter for the dis- 

 semination of the spores and as an adaptation to a terrestrial habit, and 

 that the basidium and teleutospore may have arisen from the tetraspo- 

 range in a similar and parallel series of developmental forms. 



At what stage in the evolution of the Ascomycetes from the red 

 algae the nuclear fusions in the ascus originated is not apparent from any 

 facts yet available. It may be found that some of the simpler Asco- 

 mycetes, whose asci are reported as regularly producing only four spores 

 the triple division has been replaced by the ordinary double division of 

 spore mother cells. In that case it may be expected that the fusion of 

 nuclei in the ascus will also be lacking. It is quite possible, however, 

 that this stage is not represented by any forms at present known. 



The conceptions thus developed form a consistent and harmonious 

 account of the development of the ascocarp and bring the main facts 

 as to its nature and origin into line with what has been learned in other 

 groups as to the nature of the processes of fertilization, chromosome 

 reduction, the permanence of the chromosomes and the central bodies 

 as cell structures, and the nature of the alternation of generations. 

 In these theoretical considerations I have aimed, of course, to do no 

 more than endeavor to correlate the facts as we know them to-day, and 



